Carbon is a material having excellent mechanical strengths, particularly at high temperatures and is widely used as a number of carbonaceous products in many applications such as a reducing agent for iron industry, electrode for electric furnace smelting, jet nozzle for rocket and core material for nuclear reactor. Recently, with rapid progress of atomic energy industry and aeronautical and cosmic technology, requirements for properties of carbonaceous products have become more and more severe.
Pyrolysis tars which are formed as by-products in a high temperature cracking of petroleum distillates such as naphtha and gas oil to produce ethylene and other olefins and acetylenes, e.g. those of so called "ethylene tar bottoms", contain a high proportion of polycyclic aromatic hydrocarbons and are conveniently used as raw materials of carbonaceous products. A disadvantage of such a pyrolysis residue is to absorb oxygen in the air even at room temperatures, which prevents such aromatic hydrocarbons from being further polycondensed into large crystals of a graphite structure. Therefore, it is impossible to produce a high quality needle-like coke directly from the pyrolysis residua. The production of a needle-like coke from coal origins is more difficult than that from petroleum residua because a substantial amount of oxygen is inevitably present during mainly the carbonization step of coal due to impossibility of complete shutting off of oxygen from the system.
In general, the heating of asphalt or pyrolysis tar bottoms eventually results in the formation of a solid coke due mainly to the polycondensation reaction of polycyclic aromatics. A polarized microscopic view at room temperature of samples, for the purpose of observation of the course during which the asphalt or tar bottoms initially in a liquid state under heating is gradually changed into a solid coke, shows that the heating up to about 300.degree. C. of the material gives a homogeneous phase only, but that the heating up to about 400.degree. C. gives a variety of spherical phases wherein smaller- and larger-sized spheres are intermingled together. When continuing the heating of the material up to about 450.degree. C., there is observed a well-developed stripe-pattern existing in a stratiform in the larger spheres which becomes needle-like structure when heated to 1,400.degree. C. The larger spheres gradually grow at 400.degree.-450.degree. C. whereby to integrate together to form much larger spheres with the growth of stratified stripe-structure which is visible by the naked eye if heated at 1,400.degree. C. In contrast, the smaller spheres are observed on the microscope not to grow further and not to integrate together or with the larger spheres, but to leave as they are and to disperse in the spaces between, and on the surface of, the larger spheres and whereby to prevent the larger spheres from growing into much larger ones. With a high content of such smaller spheres, the coke derived therefrom has no needle-like structure, but a black-sooty appearance when calcined at 1,400.degree. C., possibly because the smaller spheres are being dispersed in the coke in the form of small sooty particles as such.
An intermingled phase of smaller- and larger-sized spheres which appears when asphalt or pyrolysis tar is heated to a temperature of around 400.degree. C. and which is not completely solidified yet is called "mesophase". In fact, in that state, the smaller particles have been substantially solidified, whereas the larger particles almost not yet solidified, so that the latter itself is really worthy to call mesophase. This is, however, not general in the related art. Similarly, the intermingled phase as above-mentioned is called "an insoluble phase" for a number of years in the art (see, for example, U.S. Pat. No. 2,775,549 to F. L. Shea, Jr.). In fact, the formability and behavior of such smaller- and larger-sized spheres will largely depend on the nature of the starting materials, but the smaller spheres are generally liable to be formed at temperatures lower than those at which the larger spheres are formed. Thus, as a rule, the smaller spheres appear in the materials at around 350.degree. C. and the larger spheres start to be formed at around 400.degree. C. Hereinafter, I refer to the smaller spheres as "impure mesophase" and the larger spheres as "pure mesophase".
My experiences on coke formation suggest that the higher the contents of oxygen-containing impurities in starting petroleum heavy residue, the larger the formation of the smaller spheres. For example, a topped residue from Djatibarang crude oil, if heated to a temperature around 350.degree. C. for several hours, deposits a large amount of a sludgy solid phase therein due to the formation of such smaller spheres, whereas the residue from Minas crude oil does not give rise to such phenomenon. Further, the former emits a slight phenol-like odor upon being subjected to a hydrogenation, which is evidence of the presence of oxygen-containing impurities therein. The fact that the presence of such oxygen-containing impurities in a petroleum residue, if the latter is coked, would prevent the coke so formed from being so grown as to form polycondensates of graphitized structure with larger crystal sizes can be clearly appreciated by such observation on a polarized microscope of a product obtained by heating the residue at 450.degree. C. for 1 hour that impure mesophase comprising fine sooty carbonaceous particles much prevails against pure mesophase comprising larger carbonaceous spheres in an unstable, disordered structure. A sufficient growth of pure mesophase would result in the formation of dense, mechanically strong coke particles which would be converted to a graphitized form with compact lattice structure which has a low coefficient of thermal expansion. In contrast, the higher the content of impure mesophase in the material, the lower the degree of growth of pure mesophase with the decrease in the compact lattice spacing of the resulting graphitized coke, the lower the mechanical strength of the coke product and the higher the coefficient of thermal expansion thereof. Therefore, in order to make possible the production of carbonaceous products resistant to, and stable under, their working conditions which are now becoming more and more severe, it is essential to subject only pure mesophase to coking, i.e. polycondensation, by previously removing impure mesophase from the starting material even in a trace residual amount.
Various attempts have been made to remove impure mesophase from coking materials. Thus, among many methods proposed therefor, one method is based on such discovery that impure mesophase is easier to be polycondensed than pure mesophase and carbonized at temperatures as low as 350.degree..about.400.degree. C. and comprises heat-treating a coking material, i.e. a petroleum heavy residue, so as to preferentially polycondense the impure mesophase, separating the impure mesophase thus polycondensed from the pure mesophase by flash distillation so as to remove the former as pitch at the bottom of the flashing column and to recover the latter as distillate at the top of the column for coking (refer to K. Hayashi et al., U.S. Pat. No. 4,177,133). Another method for the removal of impure mesophase is to treat a coking material with activated clay or zeolite at 300.degree..about.400.degree. C. so as to adsorb easily carbonizable impure mesophase preferentially on the surface of the said treating agent (refer to U.S. Pat. No. 2,775,549 above-cited, for example). This method has such disadvantage that the complete separation of solid particles of clay, etc. from the material is difficult and thus the quality of the product, coke, is rather lowered. There are other methods based on catalytic hydrogenation of a coking material, the catalyst being platinum or molybdenum deposited on an alumina or activated carbon as carrier, where impure mesophase is hydrogen-refined (for example, refer to K. Hayashi; A Fundamental study on a Process of Manufacture for High Purity Coke, Annual Report 1978 of Coal Research Institute, Faculty of Engineering, Hokkaido University, Japan, page 67). These methods also involve a difficulty in the separation of catalyst particles.
H. O. Folkins has recently proposed in U.S. Pat. No. 3,817,853 that the coking of pyrolysis tars can be improved to give an improved yield of distillate product and a reduced yield of coke with a higher quality of the coke if the pyrolysis tars are pretreated prior to coking by a hydrogenation treatment. Although the hydrogenation treatment is generally disclosed to be capable of effecting in the presence or absence of a catalyst, there is no contrete disclosure at all about embodiments of non-catalytic hydrogenation. Further, the Folkins' U.S. patent shows that the hydrogenation is effected at mild conditions at temperatures from 250.degree. F. to about 800.degree. F. (i.e. 120.degree. to 427.degree. C.), preferably from 375.degree. to 600.degree. F. (i.e. 190.degree. to 316.degree. C.) at a pressure of from 100 to about 1,500 psig (i.e. about 7.about.105 Kg/cm.sup.2 G), preferably of from about 200 to about 1,000 psig (i.e. about 14.about.70 Kg/cm.sup.2 G), so as to consume approximately 100 to about 2,000, preferably 300 to 1,000 cubic feet of hydrogen per barrel of pyrolysis tar feedstock. Experiments in Example 1 of the Folkins' U.S. patent suggest without doubt that the hydrogenation of said process does not adopt a gradual heating procedure, but is carried out by passing the feedstock to a fixed bed catalyst maintained at a predetermined temperature, namely at 550.degree. F. (288.degree. C.) in Experiment No. 1 and at 750.degree. F. (399.degree. C.) in Experiment No. 2 with such results that Experiment No. 1 gives a much lower yield of coke with a significantly higher quality thereof than those of Experiment No. 2.